1. Field of the Invention
The present invention relates to an image on-demand transmitting device and a method thereof.
2. Description of the Related Art
Technique for Reducing the Size of an Image Encoding Device (Codec).
It has been a long time since image encoding (compression) algorithms for a videconference and a videophone were produced. An image encoding device has been reduced in size and power consumption by using LSI technology and the market become mature. Image encoding devices are no longer stationary and large.
Mobile Communications Technique
A cellular phone and a PHS (Personal Handy phone System) exhibit explosive growth in their second generation (of a digital type) as a result of a reduction of device size and the strenuous efforts to improve the infrastructure made by carriers (companies providing communications services). On a worldwide scale, there is a move afoot in the third generation to standardize methods unique to respective countries and areas.
There is a demand for implementing an image transmitting system, which is carried by a person and implemented via a mobile communications infrastructure as shown in FIG. 1. For example and as is shown in FIG. 1, when a worker 201 goes to a work site carrying a small image transmitting device 200, inspects or repairs a leak in a meter or a valve, etc., and reports the state of construction, progress checking, operations, a disaster site, etc., he or she captures an image with his or her portable small image transmitting device, and transmits the captured image to a control room 202. At the control room 202, a system such that the state of progress checking of the operations, etc., is judged according to the image received from the worker 201, and a necessary instruction is given to the worker 201 by voice, may be considered. The communications system adopted by the worker 201 is a wireless communication. With current communications systems such as a cellular phone system, etc., it is impossible not only to transmit an image of sufficiently high quality at a high resolution, but also to secure a sufficient bandwidth when a communications line is used for the above described purpose also in W-CDMA, which is currently being developed for future implementation. As described above, a (on-demand) system with which the control room 202 can issue an instruction to the worker 201 in real time in response to a transmitted image is demanded. However, since a sufficient bandwidth cannot be secured, satisfactory operability cannot be implemented even with the communications systems provided by carriers.
Unlike a system of MPEG 2, etc., the infrastructure of narrow-band communications such as PHS and ISDN systems, etc., which are available at low cost, is dominant in the existing image transmitting systems such as a videoconference, videophone, a supervisory codec, etc. Currently, the following methods are dominant for encoding systems in a narrow-band.    (1) A method for encoding an image itself, which is captured by a camera (Motion JPEG, etc.).    (2) A method for encoding a difference from an immediately preceding captured image (H.261 and H.263).
With the method (1), each image is encoded, so that the amount of information to be transmitted becomes large and each image is displayed slowly. Hence, by reducing the amount of information of each image with the use of the continuity of a moving image, the method (2) is implemented.
In the existing image transmitting systems such as a videoconference, a videophone, a monitoring codec, etc., a camera is installed. Therefore, the angle of the camera is fixed or can be changed only in the horizontal and vertical directions. Accordingly, the background of an image captured by the camera is the same as that of an immediately preceding captured image, or the whole background is moved in the horizontal or vertical direction. As a result, encoding of the (motional) difference from the immediately preceding captured image allows the amount of information to be reduced in comparison with the encoding of the image itself, which is captured by the camera, whereby a transmission efficiency can be improved.
However, since the (motional) difference is encoded, the following problems may arise when a person carries or wears a camera, and uses it.
That is, because the (motional) difference from an immediately preceding captured image is detected in the horizontal and vertical directions, image motions other than those in the horizontal and vertical directions (by enlargement, camera rotation, etc.) cannot be supported. As a result, the entire background must be redrawn.
For this reason, the transmission efficiency is degraded, and a solution to the above described problems is demanded.
This specification refers to, as a narrow-band communications system, such a communications system which cannot secure a sufficient bandwidth for an image on-demand transmission. When an image on-demand transmission is performed with such a narrow-band communications system, a possible solution is such that low-resolution images are frequently transmitted, high-resolution images are transmitted at a low frame rate, intermediate-resolution images are transmitted at an intermediate frame rate, or these transmission methods are switched depending on need.
FIG. 3 exemplifies the configuration of a conventional image on-demand communications system using a narrow-band communications system.
Images captured by a camera 207 that the worker possesses are transmitted to a codec 205 on a transmitting side, which encodes the images. However, since a communications bandwidth is not sufficient, only the minimum of required data is stored in a buffer memory 209 for encoding by discarding most of the large amount of image information obtained with the camera 207. Then, the data is encoded and transmitted. A codec 206 on a receiving side stores the data transmitted from the codec 205 on the transmitting side in a buffer memory 210 for decoding. The data is decoded and displayed on a monitor 208 in a control room, etc. Accordingly, only images of low quality can be viewed in the control room, and at the same time, most of the large amount of image information captured by the camera 207 is discarded on the transmitting side, leading to inefficiency.
The mobile communications technology in the second generation cannot secure a broad transmission line bandwidth. A broadband will not necessarily be available at low-cost even in the third generation, because frequencies appear to be used up. Therefore, an image encoding (compression) algorithm must be used in a narrow-band, so that image encoding must be performed by giving precedence to a frame rate (motions) over an image quality, or vice versa, or by making a trade-off between them.